really dig this sketchbook, is really focused on what you like, great direction, keep it up, your stuff is really cool and creamy, live those suculent edges and colors.
about the last characters, yeah i get what you mean, i had a similar problem with a character recently
on the last one, im not so sure studies are gonna help you out on this sort of subject.
i would say you need to watch out for theory and other stuff in order to solve what you dont understand, i would suggest perhaps some readings and stuff to try to understand whats failing,
have you seen carlyn lim latest challenge on artstation the one of ancient cultures?
she speaks of concept art in a way that left me quite amazed,
i suggesr to make some researches on her procss of thought when making characters if you havent already.
and if you find some cool guies on how to create good charactersdont hesitate on sharing 
